Yesterday Phoronix showed proof of Steam's Linux client existence via its Mac OS X launcher that is currently in closed beta, then this morning they showed further signs of Linux support. Since 2008 it was known that Steam and the Source Engine would come to Linux. As an update, they even pointed out the download link for the Steam Linux binary from their store.

Even though the Source engine (used in HL2, CS:2 TF2 and some other games) is capable of using OpenGL instead of DirectX, Valve will most likely port Steam UI as method of distributing Linux servers (which account to a large portion of the install based). I doubt that they will go the extra mile and port the client games to Linux (even though a MAC -> Linux port is fairly strait forward).

Though, hopefully I'm wrong. (I've got a number of Steam games that I have played in ages, since I dismantled my Windows XP machine)